Urea-formaldehyde resins are widely used in the wood products industry as adhesives for plywoods and binder resins for hardboard, chipboard, particle board and the like. Recently, due to considerations of safety and fire prevention, there has developed a demand for fire-retardant or fire-resistant plywoods and particle board or chipboard. Therefore, the prior art has sought methods for the incorporation of fire-retardant salts, and specifically such salts as ammonium phosphates and polyphosphates, into plywoods and particle board.
The prior art methods of incorporation of ammonium phosphates into particle boards have required mixing in the dry phosphate salt or spraying a solution thereof onto the particle board wood furnish and drying such furnish before adding resin binder thereto. However, a sprayed solution of ammonium polyphosphate will remain undried and sticky for long periods even at high temperatures. It requires temperatures of over 500.degree.F. to render it non-sticky and this is far higher than normal hot pressing temperatures for particle boards. Alternatively, it has been suggested to incorporate the powdered dry salt into a binder resin, but such dry powdered salts are very difficult to disperse and maintain evenly dispersed in such resin solutions. Furthermore, they interfere with the smooth operation of the furnish depositing equipment when dry mixed into such furnish.
Attempts to blend aqueous solutions of ammonium polyphosphates with aqueous syrups of urea-formaldehyde or melamine-urea-formaldehyde resins have been successful only in very low concentrations of the phosphates, since higher concentrations of phosphates cause the resins to cloud and precipitate. These low concentrations are generally not sufficient to confer an acceptable level of flame retardancy on the final particle board product. Consequently, it has been desired to incorporate aqueous solutions of phosphate salts into urea-formaldehyde and melamine-urea-formaldehyde resins at concentrations higher than 10% by weight in order to simplify the process of production of fire-retardant particle boards.
It has now been found that certain aminoplast resins will remain stable to aqueous solutions of modified ammonium polyphosphates for extended periods making possible a fire-retardant binder resin solution quite suitable for use in the general commercial process for the production of particle board. The availability of such binder resin solutions makes possible the production of fire-retardant particle board produced as rapidly with the same number of steps as are required for the present production of non-fire-retardant particle boards. These novel fire-retardant binder resin solutions can be used in the same manner and with the same facility as ordinary aminoplast binder resin solutions but contain much higher concentrations of ammonium phosphates than are compatible with such normal resin solutions. Thus, industry is enable to produce fire-retardant particle board by the same processes and at the same rate of production as the non-fire-retardant particle boards. A further advantage is the fact that the novel binder resin solutions do not require dilution with increased amounts of water which would, in turn, require a separate drying step for the wood furnish. As a consequence, these novel binder resin solutions and process for their production represent a decided improvement in both economics and safety.